This invention relates to batteries with prismatic housings, and their construction.
Prismatic cells are prismatic rather than circular in cross-section. Examples of such prismatic cells include rechargeable F6 nickel metal hydride (NiMH) cells currently used in portable electronics equipment, such as cellular phones, which have rectangular cross-sections. The housings of such prismatic cells typically include a deep drawn (or longitudinally seamed) can which is capped with a rigid, flat plate cover assembly having one or more electrical contacts. The cover is typically pressed down into the prismatic, open end of the can until the upper edge of the cover is slightly below the upper edge of the can, and then welded in place (e.g., by laser welding) to seal the cell. Housing materials include stainless steel or nickel plated cold rolled carbon steel (CRS).
Precise tolerancing of the can and cover are required to provide intimate contact about the entire periphery of the cover for a good seal upon welding. Ideally, the parts are dimensioned line-to-line or for a slight interference fit in order to avoid gaps along the weld seam. If the interference is excessive, the flat plate cover can buckle, typically through the hole or holes punched through it for the contact/vent assemblies. If the cover bends significantly, the seal at these holes is compromised. If there is an excessive local gap between the cover and can, obtaining a hermetic laser weld will be difficult, due in part to the lack of filler material used in the welding process. These concerns may be addressed by requiring extremely tight dimensional tolerances on critical can and cover dimensions, thus increasing the cost of the components. Some current F6 designs employ overall can tolerances of +/-0.002 inch (0.05 millimeter), for instance, which can be difficult to reliably, economically achieve with deep drawing.
The contact/vent assemblies are typically mounted in round holes punched through the flat plate cover, electrically isolated from and sealed to the cover with a polymeric seal. During manufacture and in use, care must be taken to avoid rotating the contact/vent assemblies within their mounting holes, which can cause internal shorting.
Particularly with thin cells, such as the common F6 prismatic cell, care must be taken not to damage or distort the seal material at the contact/vent assembly during the welding process, due to the close proximity of the edge of the seal to the weld seam. Limiting the heat in the area near the seal can reduce the distortion of the seal and reduce the reject rate due to leakage at the seal, but can also result in a higher reject rate due to leakage at the seam.
Many standard prismatic cell sizes, such as the F6, require a specific height of their contacts relative to the edge of the open end of their cans. After the cover is welded in place, spacers are sometimes added between the cover and the outer contact to control this dimension. Ideally, the covers are precisely positioned as far outward as possible to provide the most internal cell volume for active materials.